This invention relates generally to a clock positioning circuit for automatically positioning the timing pulses used for a digital data transmission system.
In electronic data transmission, the quality or reliability of the transmission system may be expressed in terms of a bit error rate, i.e., the number of erroneously received digital pulses per unit of time divided by the bit rate of the data channel. The error rate is determined by a number of factors including intersystem interference, noise, fading, equipment misadjustment and the like.
One method of improving the quality of a data transmission channel has been through the use of automatic gain control (AGC) in the receivers. Provisions are made to switch channels when the gain control feedback exceeds certain limits. While this method can detect a weak or faded signal, signal distortion due to noise is not identified and may go undetected. Another method employs the transmission of a known pattern of data as a means for checking the received data for accuracy. Of course, normal transmissions must be interrupted for transmission of the known patterns.
It is often necessary--particularly in the case of "hot standby" and diversity systems--to have a continuous rapid responding and reasonably accurate estimate of whether the bit error rate (BER) of a digital system such as a digital radio exceeds some predetermined value. This estimate should include the effects of degradation in alignment of the modulator, RF and IF circuits, the carrier recovery, phase detector and clock recovery portion of the demodulator, and the effects of distortion. Such an estimate can be obtained in the moddemod section by using a pseudo-error detector. The pseudo-error detector provides a measure of the quality of the "eye pattern" and from this measurement a close approximation of the actual bit error rate can be extrapolated. As used herein the "eye pattern" or "eye opening" refers to the image as seen on an oscilloscope in response to digital data when the horizontal sweep rate is equal to the baud, bit or clock rate. Such an oscilloscope display is widely known in the art as an "eye pattern" due to its resemblance to the human eye. Reference may be made to U.S. Pat. No. 3,721,959. Also, in this context, the term "mod-demod section" is used to describe a section where the eye pattern and the BER are directly related. (A single pseudo-error detector can only monitor what takes place between data regenerators.)
For further discussion relating to error detection, reference may be made to: D. J. Godding, "Performance Monitor Techniques for Digital Receivers Based on Extrapolation of Error Rate", IEEE Transactions on Communication Technology, Vol. COM-16, pp. 380-387, June 1968; S. B. Weinstein, "Estimation of Small Probabilities by Linearization of the Tail of a Probability Distribution Function," IEEE Transactions on Communication Technology, Vol. COM-19, pp. 1149-1155, December 1971; and, B. J. Leon, H. L. Hammond, Jr., P. A. Vena, W. E. Sears, III and R. T. Kitahara, "A Bit Error Rate Monitor for Digital PSK Links," IEEE Transactions on Communications, Vol. COM-23, pp. 518-525, May, 1975, which detail analytical studies of the subject, especially when related to FSK, BPSK and WPSK signals, and broadly to all forms of digital transmission.
In the above referenced U.S. Pat. No. 3,721,959 an approach to pseudo-error monitoring was disclosed which uses a dual voltage comparator to compare an eye pattern with an established reference voltage and clock pulse having a selected pulse width that is used to define a rectangular boundary within the eye pattern such that each violation of the boundary is detected and counted as a pseudo-error. When the signal to noise ratio is high and the signal is not degraded by transmission distortion, the eye pattern will not violate the window. Conversely, any combination of noise and distortion of sufficient magnitude can cause the baseband signal to become distorted so that the window is violated and the baseband signal voltage at the input to the comparator falls between two established thresholds, which results in the outputs of the two comparators to simultaneously change states. A Boolean AND function is performed upon the two outputs of the voltage comparator to give a logic one pulse for each window violation which is counted as a pseudo-error.
The pseudo-error detector is very useful for an effective channel indicator on a multi-channel system, where an indication that a preselected number of errors have been received on a message channel for a given period of time will enable the determination that the message channel is defective. It is thus a relatively easy matter to switch to an alternate channel with a less predominant bit error rate.
Although the above references provide techniques for detecting error rates and pseudo-error rates, they fail to provide a means for minimizing the bit error rate without switching modes of operation or communication channels. It has been found that a minimum bit error rate (BER) can be achieved by centering within the eye pattern the timing pulses that are used to synchronize the received data with the data detection circuits.